The logic behind the President-elect's supposed interest in recruiting Senator Clinton into his Cabinet is allegedly to be found in Doris Kearns Goodwin's Team of Rivals, the best-selling account of Abraham Lincoln's Civil War Cabinet. Lincoln recruited numerous GOP power-brokers into his orbit, including defeated rivals for the 1860 nomination, and Goodwin will tell you (I haven't read the book) that it all went well.
If anything, it would be less risky for Obama to assemble such a cabinet than it was for Lincoln. In Abe's time, there was nothing to stop Cabinet members from scheming to succeed or even to overthrow the President in the next election. Lincoln was in fact giving people like William Seward and Salmon P. Chase stronger positions to challenge him from, had they chosen that course. During our "early national period," a Cabinet post was the strongest credential for a potential President. Once Thomas Jefferson's party took over in 1801, in fact, the Secretary of State, rather than the Vice President, was the heir apparent to supreme power. James Madison was Jefferson's Sec. of State and succeeded him, and Madison's Sec., James Monroe, succeeded him. The pattern repeated itself once more with John Quincy Adams, but Monroe's Cabinet was probably the most formidable "Team of Rivals" ever. Along with Adams at State, it included two other men who would vie with Adams for the presidency in 1824: Treasury Secretary William Crawford and War Secretary John C. Calhoun. None would defer to another, and Crawford actually won the party nomination via a congressional caucus. Adams ran regardless, while Calhoun hedged his bets by serving as running mate to both Adams and Andrew Jackson. The consequence of Monroe packing his Cabinet with such ambitious talent was that the country went from the "Era of Good Feeling" and a de facto one-party system to the disintegration of Jefferson's party in just eight years.
Some observers understandably worry that Senator Clinton might act as if she were in the Monroe Cabinet rather than Lincoln's, but I think that the parties, not to mention the public, expect more deference to the President from his department heads. Clinton could not hope to hold the job, if she gets it, if she appeared to use it to build a campaign organization, even if it wasn't meant to go operational until 2016. Furthermore, it's been one hundred years since a candidate went directly from the Cabinet to the Presidency; the last man was Secretary of War William Howard Taft in 1909, and standards were so different back then that no one really cared that the Presidency was Taft's first elected office. Following this logic, bringing Clinton into the White House would be a way of "keeping your friends close and your enemies closer. But calculations of that kind shouldn't outweigh Clinton's unsuitability for the office.
As for Senator McCain, his meeting with Obama today provoked speculation that the Republican might be made part of the team. But even if Obama can work something out with McCain, the Arizonan would probably be of more use to him right where he is, as a potential filibuster-buster in the Senate. And with that, speculation about a "team of rivals" should end, since no other Democrat can be taken seriously as a rival who needs to be incorporated into the Cabinet. The only other gambit I can imagine would be for Obama to recruit Mike Huckabee or Mitt Romney, his most likely Republican rivals four years from now, and neither man would have any reason to accept such invitations were they offered. By process of elimination, the only motive for appointing Clinton, absent the emergence of a real team of rivals, would be to neutralize her as an intra-party threat, and that would be an irresponsible motive for a potentially disastrous decision.